1. Field of the Invention
This invention relates to the detection of targets in an interrogation zone and more paticularly it concerns novel methods and apparatus for identifying a characteristic signal produced by special magnetic targets mounted on books or merchandise as they are carried through an interrogation zone at the exit from a protected area.
2. Description of the Prior Art
French Pat. No. 763,681 dated May, 1934 discloses an electronic detection system for detecting the unauthorized taking of books or merchandise from a protected area. According to the French Patent the books or merchandise have affixed thereto "targets" in the form of a strip of a high magnetic permeability material characterized by magnetic saturation at low induction. One such material is known by the name of permalloy. As described in the French patent, transmitting and receiving antennas are set up at an exit from the protected area. The transmitting antenna is energized to generate an alternating magnetic interrogation field in an interrogation zone at the exit. When an article carrying a target is brought through the zone the alternating magnetic field drives the target into and out of magnetic saturation. The target in turn, produces characteristic electromagnetic disturbances in the form of pulses which are made up of harmonics of the magnetic interrogation field frequency. The receiving antenna is arranged to receive these pulses and a receiving apparatus is connected to the receiving antenna to respond to selected ones of the harmonic frequencies produced by the target.
A problem that occurs in a detection system of the type described above is that of discriminating between true targets and other pieces of metal or magnetic material that might be carried through the interrogation zone. In order to provide a magnetic interrogation field which is strong enough at a distance of, for example, two feet (60 cm.) or more from the interrogation antenna to drive the target into saturation, the magnetic field must be so strong in the immediate vicinity of the antenna that it will also drive many ordinary metal objects into saturation and cause them also to emit harmonics of the interrogation field frequency.
French Pat. No. 763,681 points out than by arranging, in the object which may be stolen, a magnetized metal part, one can detect the presence of this part by the harmonics of even rank which appear in such case. The same patent also suggests passing into the antenna a direct current superimposed on the alternating current to modify the initial permeability of the target. U.S. Pat. No. 4,326,198 also discusses the use of a separate bias field antenna next to the interrogation antenna to cause the target to produce even harmonics of the interrogating field frequency. The same patent further discloses that the earth's own magnetic field can be used to bias the target so that it will produce a predominance of even harmonic frequency components. U.S. Pat. No. 4,384,281 also discloses an electromagnetic type theft detection apparatus which incorporates signal gates and noise gates and comparison means for comparing signals of different frequencies and signals which occur at different times.
It so happens that the presence of the earth's magnetic field also causes ordinary metal objects to produce even harmonic frequency components when such objects are driven repetitively into and out of magnetic saturation. Accordingly, it is not always possible, simply by detecting only even harmonic frequencies, to distinguish between various metal objects and the targets themselves.
A further problem found in the prior art is that electromagnetic fields from other sources are present in the interrogation zone and these other fields can interfere with and overwhelm the fields produced by the targets. These other fields are random in amplitute, frequency and phase; and they are difficult to eliminate without eliminating the true target signals.